The present invention relates to spread spectrum communications systems, and more particularly, to transmission and reception processing of information signals within spread spectrum communications systems.
In code division multiple access (CDMA) communications systems, both direct sequence (DS) systems and frequency hopping (FH) systems, it is often desirable that signal transmitters and receivers be capable of accommodating variable data throughput rates. For example, CDMA transmitters and receivers are often used to communicate digital packet data for multiple logical and physical channels, each channel possibly requiring a different data transmission rate. Additionally, CDMA transmitters and receivers sometimes must alternately convey both packet data and continuous data.
Thus, if a CDMA transmitter or receiver is not configured to accommodate varying data throughput rates, overall system quality can be significantly degraded. For example, if logical channels of varying data rate are input to a constant data rate CDMA transmitter, buffering in the transmitter will lead to signal delay during periods when the data input rate exceeds the constant data rate of the transmitter. Alternately, when the data input rate drops below the constant data rate of the transmitter, the transmitter must either fill outgoing data frames with dummy bits, thereby reducing system capacity, or resort to discontinuous transmission (DTX), which can lead to spectral jamming of proximate equipment.
Accordingly, recent spread spectrum systems have been designed specifically to accommodate varying data throughput rates. For example, the recently developed Universal Mobile Telecommunications System (UMTS)/International Mobile Telecommunications in the year 2000 (IMT-2000) standards include provisions for variable data rate transmission. See, for example, E. Dahlman et al., UMTS/IMT-2000 Based on Wideband CDMA, IEEE Communications Magazine, Vol. 36, No. 9, September 1998, pp. 70-80, which is incorporated herein in its entirety by reference. The UMTS/IMT-2000 standards are also described in detail in a number of technical specifications put forth by the well known 3rd Generation Partnership Project (3GPP(trademark)).
While such variable data rate systems do overcome the above described problems of signal delay, reduced system capacity and discontinuous transmission, they can introduce other difficulties. For example, in the above described IMT-2000 and other standards, variable data rates are achieved by applying a variable per-packet spreading factor (e.g., variable-rate data symbols are spread using constant rate pseudo-noise, or PN, spreading sequences), and the spreading factor used for each particular data packet is included in, and transmitted with, the data packet itself. However, the spreading factor is typically transmitted via a number of rate indication symbols which are spread throughout the data packet, and the spreading factor therefore cannot be determined at a receiver until the end of the data packet. Consequently, known receivers must collect an entire incoming data packet before despreading can begin. As a result, known variable data rate despreaders include very large memory buffers and are therefore rather costly and inefficient. Thus, there is a need for improved methods and apparatus for transmitting and receiving variable data rate information signals in spread spectrum communications systems.
The present invention fulfills the above-described and other needs by providing CDMA spreading and despreading techniques which eliminate the need for a frame-length chip buffer within the despreading and demodulation processor of a CDMA receiver. According to the invention, successive CDMA frames are transmitted with variable spreading factors and a constant spreading sequence chip rate to provide a variable data throughput rate. Thus, as in many well known CDMA standards (e.g., the above described UMTS/IMT-2000 standard), each CDMA frame includes a variable number of data symbols and a variable number of rate indication symbols which are transmitted in slots throughout the frame.
According to the invention, however, channelization spreading sequences used for higher data rate frames (i.e., frames having lower spreading factors) are guaranteed to be subsets of channelization spreading sequences used for lower data rate frames (i.e., frames having higher spreading factors). As a result, a CDMA despreading processor can despread incoming data symbols as they arrive at a CDMA receiver, rather than having to collect all of the incoming PN chips for a frame before despreading the frame.
Specifically, a CDMA despreader according to the invention despreads incoming chips using a minimum allowable spreading rate, and the resulting despread data symbols are stored to memory (e.g., within a deinterleaver or error detection and correction processor in the CDMA receiver, which typically require frame-length data symbol buffers, irrespective of operation of the despreader and demodulator). Then, at the end of the incoming frame, the received rate information symbols are decoded to determine the actual spreading factor used in spreading the frame during transmission. If it is determined that the frame was actually spread using the minimum allowable spreading factor, then the stored data symbols are accepted as the finally despread data symbols, and deinterleaving and error detection and correction decoding is carried out directly using the stored data symbols. Otherwise, the stored data symbols are taken to be temporary, or soft, data symbols which are then combined, using the guaranteed relationships between fast and slow spreading sequences, to generate the finally despread data symbols. Advantageously, the conversion from temporary data symbols to finally despread data symbols can be carried out within the deinterleaver or error detection and correction processor. As a result, a CDMA receiver according to the invention can operate successfully without a frame-length chip buffer and is therefore less expensive and more efficient as compared to conventional receivers.
According to the invention, an exemplary code division multiple access transmitter for transmitting a succession of source data frames, each source data frame including a sequence of source data symbols, and each source data frame being intended for a distinct recipient, includes a spreading and modulation processor configured to spread each source data symbol within a source data frame, using any one of a plurality of predefined spreading sequences, to provide a spread spectrum signal for transmission to one or more spread spectrum receivers. According to the invention, each spreading sequence provides one of a plurality of possible spreading factors. Additionally, rate information is included in each transmitted data frame to indicate a spreading factor used in spreading the transmitted data frame, and first, higher-order spreading factor used in spreading data frames for a particular recipient is guaranteed to be an arithmetic combination of multiple copies of a second, lower-order spreading factor used in spreading data frames for the particular recipient.
An exemplary method for transmitting a succession of code division multiple access source data frames, each source data frame including a sequence of source data symbols, and each source data frame being intended for a distinct recipient, includes the steps of: spreading each source data symbol within a source data frame, using any one of a plurality of predefined spreading sequences, to provide a spread spectrum signal for transmission to one or more spread spectrum receivers, each spreading sequence providing one of a plurality of possible spreading factors; including rate information in each transmitted data frame to indicate a spreading factor used in spreading the transmitted data frame; and guaranteeing that a first, higher-order spreading factor used in spreading data frames for a particular recipient is an arithmetic combination of multiple copies of a second, lower-order spreading factor used in spreading data frames for the particular recipient.
According to the invention, an exemplary code division multiple access receiver, includes: a despreading processor for despreading an incoming spread spectrum data frame based on a minimum allowable spreading factor; and a memory for storing data symbols resulting from the despreading based on the minimum allowable spreading factor. The incoming data frame includes rate information indicating an actual spreading factor used in transmitting the incoming data frame, and the data symbols stored in the memory as a result of the despreading based on the minimum allowable spreading factor are combined to provide refined data symbols when the actual spreading factor is determined to be greater than the minimum allowable spreading factor. The exemplary receiver can further include a deinterleaver, and the deinterleaver can be used as the memory for storing data symbols.
A exemplary method of receiving spread spectrum data frames, each data frame including rate information indicating an actual spreading factor used in transmitting the data frame, includes the steps of: despreading an incoming spread spectrum data frame using a minimum allowable spreading factor; storing data symbols resulting from despreading the incoming data frame to a memory; decoding the rate information included in the incoming data frame to determine the actual spreading factor; and combining the data symbols stored in the memory to provide refined data symbols when the actual spreading factor is determined to be greater than the minimum allowable spreading factor. The memory can be, for example, a deinterleaver in a spread spectrum receiver.
The above-described and other features and advantages of the invention are explained in detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those of skill in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein.